Crest factor reduction in multicarrier transmission schemes

ABSTRACT

A multicarrier transmission system uses a set of carriers spaced apart in frequency with a number of bits being assigned to each carrier. A transmitter has a mapper which maps a data signal to a parallel set of constellation values. A frequency domain-to-time domain transform stage converts the set of modulated carriers to a time-domain signal. A peak detector detects when the time-domain signal exceeds a predetermined criterion. A constellation modifier modifies the constellation value of at least one of the carriers to reduce the crest factor of the transmitted signal. A carrier is selected for modifying on the basis of a number of bits allocated to that carrier. The constellation modifier can select an alternative constellation value by an iterative method or by calculation. The constellation modifier can operate entirely in the time-domain.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to multicarrier transmission schemes such asDiscrete Multitone (DMT) and Orthogonal Frequency Division Multiplexing(OFDM) as well as to transmitter, receiver, and/or transceiver apparatusand methods.

2. Discussion of the Related Art

Multicarrier transmission schemes such as DMT and OFDM are becomingwidely used in such areas as Digital Subscriber Line (xDSL), DigitalAudio Broadcasting (DAB), Digital Video Broadcasting (e.g. DVB-T, DVB-S,DVB-H) and wireless Local Area Networks. Multicarrier transmissionschemes have many advantages, including high spectrum efficiency,resistance to interferers and noise and resistance against multipathinterference. One less desirable property of multicarrier transmissionschemes is that the transmitted signal has a very high crest factor. Thecrest factor of a signal is defined as the ratio of the peak amplitudeof the signal to the Root Mean Square (RMS) value of the signal. This isalso known as Peak to Average Ratio (PAR). The high crest factor poseschallenges for the analog front-end design of a multicarrier transmitterand increases the power consumption of the front-end, largely due to theneeds of the power amplifier.

One known solution to reduce crest factor is to clip the signal, beforetransmission. Indeed, the digital to analog converter (DAC) in amulticarrier transmitter may inherently clip the signal if the signalexceeds the range of the DAC. Clipping has a disadvantage of distortingthe signal, which can result in erroneously decoded data symbols andhence bit errors at a receiver. In frequency division multiplexedsystems, such as digital subscriber line (xDSL), where signals for theupstream and downstream paths are frequency multiplexed, clipping in onepath can cause errors in the other path. Another known solution to limitcrest factor is to use pulse shaping techniques, which can reduce theharsh effects of clipping. However, if pulse-shaping is used there is aneed for extra filtering in the analog front-end of receivers to removethe out-of-band distortion which pulse-shaping incurs and this alsoincreases the cost of transmitters. A further known method to reducecrest factor reserves certain ones of the carriers (tones) to createsymbols with lower crest factor but this has the disadvantage ofpermanently decreasing the data rate. A further known method reservessome bits for messaging between transmitter and receiver in order toproperly decode the modified symbols (with lower crest factor). Thisalso has a disadvantage of a permanently decreased data rate andrequires compatible receivers.

U.S. Pat. No. 6,757,299 describes peak power to average power reductionin multicarrier communication systems. A subcarrier symbol is identifiedwhich has maximum effect on a peak in a frame and the symbol on thiscarrier is modified to reduce the peak. This method concentrates solelyon reducing peak size, which could incur a high or unacceptable biterror rate. Once a peak has been detected, the method requires a newsymbol to be computed and then transformed from the frequency domain tothe time-domain.

The present invention seeks to provide an improved way of reducing thecrest factor in a multicarrier transmission scheme.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide improvedmulticarrier transmission schemes such as Discrete Multitone (DMT) andOrthogonal Frequency Division Multiplexing (OFDM) as well astransmitter, receiver, and/or transceiver apparatus and methods.

Accordingly, a first aspect of the present invention provides atransmitter for a multicarrier transmission system which uses a set ofcarriers spaced apart in frequency, a number of bits being assigned toeach carrier, the transmitter comprising:

an input for receiving a data signal for transmission;

a mapper arranged to map the data signal to a parallel set ofconstellation values, where each constellation value determinesmodulation of one carrier in the set of carriers;

a frequency domain-to-time domain transform stage arranged to convertthe set of modulated carriers to a time-domain signal;

a peak detector arranged to detect when the time-domain signal exceeds apredetermined criterion;

a constellation modifier arranged to modify the constellation value ofat least one of the carriers, wherein the constellation modifier isarranged to select a carrier for modifying on the basis of a number ofbits allocated to that carrier.

Selecting a carrier for modifying on the basis of the number of bitsallocated to the carrier minimizes the impact on BER. The error which iscaused by modifying the constellation value can be concealed by errorcorrection coding applied to the data signal. Rather than clipping (andhence corrupting) many carriers, only one or more carriers are lost in acontrolled manner. By selecting carriers in this way, the total impactof the clipping is significantly reduced. There is not a permanentreduction in data rate, and no clipping distortion is introduced in theproper receive path. A reduced crest factor lowers the requirements andpower consumption for the analog front-end, particularly the poweramplifier. Modifying a transmitter in this manner has no impact on thereceiver. One or more carriers can be selected and modified, and thenumber of selected carriers can be modified during use.

Preferably, the constellation modifier is arranged to select a modifiedconstellation value which will place a peak of opposite polarity at, ornear to, the position of a peak in the time-domain signal. Somestandards which define transmission schemes require a transmitter toalways transmit a valid constellation value while others allow moreflexibility. The constellation modifier can be arranged to always modifythe constellation value to a valid value, i.e. the modified value is avalid value within the constellation, even though the modifiedconstellation value will no longer correspond correctly to the datawhich was intended to be mapped to that carrier. Alternatively, theconstellation modifier can be arranged to modify a carrier to anon-valid value. This can have an advantage of better aligning theposition of a peak in the modified carrier with the peak (of oppositepolarity) in the multicarrier signal.

The constellation modifier can be arranged to select a plurality ofpredetermined alternative constellation values and the peak detector candetermine the best one of the alternative constellation values based onthe effect of the alternative constellation value on the time-domainsignal. Preferably the predetermined alternative constellation valuesare the constellation values of largest amplitude, which are to befound, at the four ‘corners’ of the constellation, when theconstellation is plotted as a constellation diagram. The modifiedconstellation value can be selected by an iterative method, whichrepeatedly modifies a constellation value and determines the effect ofeach alternative constellation value on the overall signal.Alternatively, a constellation value can be modified by calculating theposition, in time, of a peak in the selected carrier.

In an alternative form, the constellation modifier can be arranged tomodify the constellation value of at least one of the carriers in thetime-domain, without the need to re-perform the frequency domain-to-timedomain transform.

A further aspect of the invention provides a method of reducing thecrest factor of a transmitted signal in a multicarrier transmissionsystem which uses a set of carriers spaced apart in frequency, eachcarrier being assigned a number of bits, the method comprising:

receiving a data signal for transmission;

mapping the data signal to a parallel set of constellation values, whereeach constellation value determines modulation of one carrier in the setof carriers;

applying a frequency domain-to-time domain transform to the set ofmodulated carriers to generate a time-domain signal;

detecting when the time-domain signal exceeds a predetermined criterion;

modifying the constellation value of at least one of the carriers,wherein a carrier is selected for adjustment on the basis of a number ofbits allocated to that carrier.

The selection of the carrier for adjustment may be one having the fewestbits allocated to it. The selection can, for example, be of at least twocarriers having the fewest bits allocated to them. The selection can beof a plurality of predetermined alternative constellation values and itcan be determined in the method which is the best one of the alternativeconstellation values based on the effect of the alternativeconstellation value on the time-domain signal. In the method thepredetermined alternative constellation values can be the constellationvalues of largest amplitude. The modification of the constellation valuecan be selected to be only to a valid constellation value. In the methoda modified constellation value for the selected carrier can be used andthe frequency domain-to-time domain transform convert the modified setof modulated carriers to a time-domain signal after each modification.In the method the selection of a modified constellation value can besuch that it places a peak of opposite polarity at, or near to, theposition of a peak in the time-domain signal. In the method themodification of the constellation value of at least one of the carrierscan be carried out in the time-domain. In the method a time-domainrepresentation of a carrier can be stored or buffered and the selectedcarrier can be removed from the time-domain signal and added to thestored time-domain representation of a carrier to the signal at arelative phase which will place a peak of opposite polarity at, orsubstantially at, the position of a peak in the time-domain signal. Theremoval of the selected carrier from the time-domain signal can beperformed by subtracting a scaled and time-shifted version of thetime-domain representation of the carrier from the signal.

A further aspect of the present invention provides a transmitter for amulticarrier transmission system, comprising:

an input for receiving a data signal for transmission;

a mapper arranged to map the received data signal into a set of complexdata symbols, where each data symbol determines modulation of onecarrier in a set of carriers which are spaced in frequency;

a frequency domain-to-time domain transform stage arranged to convertthe set of modulated carriers to a time-domain signal;

a peak detector arranged to detect when the time-domain signal exceeds apredetermined criterion;

a constellation modifier arranged to modify the constellation value ofat least one of the carriers in the time-domain.

The present invention also provides a method of reducing the crestfactor of a transmitted signal in a multicarrier transmission systemwhich uses a set of carriers spaced apart in frequency, comprising:

inputting a data signal for transmission;

mapping the received data signal into a set of complex data symbols,where each data symbol determines modulation of one carrier in a set ofcarriers which are spaced in frequency;

performing a frequency domain-to-time domain transform to convert theset of modulated carriers to a time-domain signal;

detecting when the time-domain signal exceeds a predetermined criterion;

modifying the constellation value of at least one of the carriers in thetime-domain.

Modifying the multicarrier symbol in the time-domain has an advantagethat no further IFFT operations are required, which can reduce thecomplexity of the processing at the transmitter and therefore cost ofthe transmitter.

The multicarrier transmission system can be a system which uses a set oforthogonal carriers, such as Discrete Multitone (DMT) or OrthogonalFrequency Division Multiplexing (OFDM) but is not limited to suchschemes.

Any of the functionality described here can be implemented in software(e.g. instructions executed by a processor), hardware or a combinationof these. Accordingly, another aspect of the invention provides softwarefor controlling operation of a transmitter. The software may beinstalled on the transmitter at the time of manufacture orcommissioning, or it may be installed onto an existing transmitter at alater date as an upgrade. The software may be stored on an electronicmemory device, hard disk, optical disk or other machine orcomputer-readable storage medium. The software may be delivered as acomputer program product on a machine-readable carrier or it may bedownloaded directly to the transmitter via a network connection.

Accordingly, the present invention can take the form of a computerprogram product in a computer readable memory for controlling aprocessor to allow reduction of the crest factor of a transmitted signalin a multicarrier transmission system which uses a set of carriersspaced apart in frequency, each carrier being assigned a number of bits,the computer program controlling the processor to:

receive a data signal for transmission;

map the data signal to a parallel set of constellation values, whereeach constellation value determines modulation of one carrier in the setof carriers;

apply a frequency domain-to-time domain transform to the set ofmodulated carriers to generate a time-domain signal;

detect when the time-domain signal exceeds a predetermined criterion;

modify the constellation value of at least one of the carriers, whereina carrier is selected for adjustment on the basis of a number of bitsallocated to that carrier.

Also, the present invention can take the form of a computer programproduct in a computer readable memory for controlling a processor toallow reduction the crest factor of a transmitted signal in amulticarrier transmission system which uses a set of carriers spacedapart in frequency, the computer program product controlling theprocessor to:

input a data signal for transmission;

map the received data signal into a set of complex data symbols, whereeach data symbol determines modulation of one carrier in a set ofcarriers which are spaced in frequency;

perform a frequency domain-to-time domain transform to convert the setof modulated carriers to a time-domain signal;

detect when the time-domain signal exceeds a predetermined criterion;

modify the constellation value of at least one of the carriers in thetime-domain.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 shows a multicarrier transmission system;

FIG. 2 shows a multicarrier transmitter for use in the system of FIG. 1;

FIG. 3 shows a time-domain representation of a multicarrier signal;

FIG. 4 shows an example allocation of bits to carriers in a multicarriertransmission scheme;

FIG. 5 shows a constellation diagram of a modulation scheme which can beapplied to a carrier;

FIG. 6 shows a first embodiment of a transmitter which can modify aconstellation value;

FIG. 7 shows a time domain representation of a single one of thecarriers used in the transmission scheme;

FIG. 8 shows a second embodiment of a transmitter which can modify aconstellation value;

FIGS. 9 and 10 show performance of the improved schemes in accordancewith the present invention.

DETAILED DESCRIPTION

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. Where the term “comprising” is used in thepresent description and claims, it does not exclude other elements orsteps. Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

FIG. 1 shows the basic elements of a multicarrier transmission system. Adata signal 2 is applied to a transmitter 3 that uses a multicarriertransmission scheme such as DFT or OFDM. The modulated signal 4 istransmitted over a communications channel 5 and received at a receiver7. The communications channel can be any suitable channel, e.g. wired orwireless or cordless, optical fiber, radio, microwave, diffuse or beamedinfra-red, etc. At the receiver, 7, the received signal is demodulatedand decoded to generate an output signal 8. The transmission system canbe broadcast or unicast (point-to-point) type. During transmission,disturbances 6 and noise can corrupt the transmitted signal 4. A levelof error coding is applied to the signal 2 at the transmitter 3 to copewith the expected amount of disturbance expected during transmission toachieve a suitable bit error rate (BER) in the output signal 8.

FIG. 2 shows a block diagram of a conventional multicarrier transmitter3. Multicarrier transmission systems use a set of carriers, i.e. a setof carriers which are regularly spaced apart in frequency and which donot interfere with one another. Each carrier—which can also be called a‘sub-channel’ or a ‘tone’—is individually modulated. The number ofcarriers that are used, the type of modulation that is applied to eachcarrier, and the error correction coding together determine the overalldata rate and achievable bit error rate of the system. A data stream 2is applied to an input 10 of the transmitter 3. Optionally source codingof the data stream can be applied, e.g. to speech. A first block 12applies error detection coding and/or error correction coding to thedata stream 11. The error detection coding and/or error correctioncoding serves to improve the resilience of the data stream to errorsduring transmission. Any suitable error detection coding can be used,e.g. checksum. Any suitable error correction coding can be used, suchas: Hamming code, Reed-Solomon code, Reed-Muller code, Binary Golaycode. Optionally channel coding may be applied. Any suitable channelcoding may be used, e.g. block code, convolutional code, turbo code.Optionally, additional techniques may be applied to improve theresilience to channel distortions and/or interference, e.g.interleaving. Any suitable combination of all or some of source coding,error detection and/or error correction coding, channel coding andinterleaving can be used.

The coded signal 13 is applied to a mapper/encoder 14. Here, the data 13is mapped to a parallel set of complex data words. Each of the datawords defines the modulation, at that point in time, df one of the setof frequency carriers. A first stage (not shown) of mapper 14 separatesa serial steam of data 13 into parallel data words, one (or more) datawords per carrier. As will be described in more detail in FIG. 5, eachcarrier is modulated by a complex value selected from a constellation ofpossible values, with the selected constellation value corresponding tothe data word that needs to be transmitted. This is typically a form ofQuadrature Amplitude Modulation (QAM). Codeword vectors 15 (sometimesreferred to as ‘frequency domain encoded data’ or ‘constellationvalues’) output from the mapper 14 are then transformed using a suitablefrequency domain to time domain transform. For example, an Inverse FastFourier Transform processing block 18 can be used but the presentinvention is not limited thereto. This transforms the set offrequency-domain data to the time-domain. The IFFT processing block 18outputs samples of an output signal in the time domain. The set ofmodulated carriers 19, which have been modulated according to oneparallel set of data words and hence constellation values, is known as amulticarrier (OFDM/DMT) symbol. In a subsequent symbol, the set ofcarriers will be modulated by a different set of constellation values.Other processing may occur at block 20 and the processed signal isapplied to a digital-to-analog converter (DAC) 23 for conversion to ananalog signal, before being applied to other analog front-end componentssuch as an optional upconverter and a power amplifier 24. The resultinganalog signal 4 is output for transmission. In this embodiment the errorcorrection coding is applied within transmitter 3 by block 12.Alternatively (or additionally) the signal 2 arriving at transmitter 3can already include a certain level of error correction coding.

FIG. 3 shows a time-domain representation 20 of a signal output from theIFFT block 18. This signal represents the sum of individual time-domainsignals which each correspond to one of the carriers. It is this signalwhich can have a large crest factor. The time domain representation 30has a signal level that oscillates about a mean signal level 32, withthe oscillations dipping aperiodically to a negative peak level 34 andaperiodically rising to a maximum (positive) peak level 36. The maximum(positive) peak level 36, or the negative peak level 34, potentiallyrepresents a sample in the symbol that will cause clipping in the DAC23, with the peak typically having a short time duration 38. As will beappreciated, the ratio of the peak 36 to a root mean square (RMS) value33 represents the crest factor (PAR) for a DMT signal.

As noted above, it is not necessary to allocate the same number of bitsto each carrier in a multicarrier transmission scheme. FIG. 4 shows anexample bit allocation to carriers for the upstream channel of an ADSLsystem. There are a total of 32 carriers, with carriers 6 through to 31being used to carry data. Generally, it is found that the carriers atthe upper and lower sides of the spectrum are limited byinter-symbol-interference (ISI) and have lower SNR than the othercarriers. In the example shown in FIG. 4 the bit loading across thecarriers is 322 bits, i.e. one OFDM/DMT symbol comprises 322 bits. Thebits (b) are allocated as 2×2b (carriers 6, 31)+2×6b (carriers 7,30)+1×9b (carrier 29)+1×10b (carrier 8)+1×12b (carrier 28)+3×13b(carriers 9, 26, 27)+4×14b (carriers 10, 23, 24, 25)+12×15b (carriers11-22).

FIG. 5 shows three possible QAM constellations 40, 50, 60 that can beused to modulate a carrier. Although the three schemes are showntogether in this diagram for comparison, only one of theseconstellations would be used to modulate a single carrier. Constellation40 has four possible values (4 QAM or QPSK), constellation 50 has 16possible values (16 QAM) and constellation 60 has 64 possible values (64QAM). During each OFDM/DMT symbol, a carrier will be modulated with oneconstellation value selected from the set of possible constellationvalues available to that constellation. For those carriers which onlyneed to carry a small number of bits, constellation 40 can be used,while for those carriers which are required to carry a higher number ofbits constellations 50 or 60 could be used. Although this example showsa maximum constellation size of 64, it will be appreciated thatmodulation schemes with greater constellation sizes can be used. Also,although not shown, the constellation for a carrier which carries only 1bit will have just two constellation positions opposite one another(e.g. similar to positions 41, 43).

An improved transmitter in accordance with one embodiment of theinvention will now be described with reference to FIG. 6. The apparatusis generally as previously shown and described with reference to FIG. 2.A peak detector 26 is positioned after the IFFT block 18 and isresponsive to the time-domain multicarrier symbols 19 output by the IFFTblock 18. These multicarrier symbols, in the time domain, have the formshown in FIG. 3, with occasional peaks 38 and 34. Peak detector 26monitors the output of the IFFT block 18 and determines when the outputsignal exceeds predetermined criterion, e.g. the signal level exceeds acertain amplitude (in a positive or negative direction). FIG. 3 showstwo example threshold levels 35A, 35B which may be used by the peakdetector 26. When a peak is detected (e.g. signal level rises abovethreshold level 35A or falls below threshold level 35B), the peakdetector 26 instructs the mapper 14 to modify the mapping for the symbolin which the peak occurred. The mapper 14 is aware of the bit loading oneach of the carriers. Any suitable scheme can be used to derive theinformation about the number bits allocated to each carrier. To give afew examples, this information can be derived during initialization ofthe communication link, it can be carried in a message header or it canbe defined in a standard.

One of the carriers having the lowest number of bits is selected as acandidate for modifying. In the example bit loading shown in FIG. 4,this will be carrier 6 or carrier 31. In the simplest embodiment, theconstellation value of that carrier is modified to four alternativeconstellation positions. For each alternative constellation position ofthat carrier, the IFFT 18 is performed again and the resultingtime-domain symbol monitored by the peak detector 26. It is likely thatone of these alternative constellation positions will reduce the peak toan acceptable level. The symbol having the lowest peak is selected forfurther processing and transmission. The new constellation value nolonger correctly relates to the data word that was applied to the mapperfor that carrier and can, or will, cause a bit error at receiver 7.However, some data streams are naturally resilient to errors, e.g.speech and where error correction coding 12 has been applied to thesignal, this can accommodate a certain level of bit errors and theoverall benefit to the BER is significantly greater than clipping theentire multicarrier symbol. The number of possibilities to create a newsymbol depends on the constellation size of the carrier to be modified.In accordance with an embodiment of the present invention not all ofthese alternatives are tried. For example, it is preferred that thealternative constellation values selected in this method are at the fouroutermost corners of the constellation diagram. For the QPSKconstellation, these are points 41, 42, 43, 44 (which happen to be theonly positions in this constellation). For the 16QAM constellation, thealternative constellation values are points 51, 52, 53, 54 (out of the16 total constellation positions). For the 64QAM constellation, thealternative constellation values are points 61, 62, 63, 64 (out of the64 total constellation positions.) The outermost constellation positionsare chosen as these are the constellation positions that have greatestsignal amplitude are likely to have maximum effect on the peak in themulticarrier symbol. This process of finding alternative constellationvalues requires a maximum of 4 extra IFFT operations—one IFFT peralternative constellation, value. If, following this method, thetime-domain multicarrier symbol still has an unacceptably high peak, asecond carrier is selected having the second fewest bits allocated to it(e.g. the other one of carriers 6, 31 not selected on the firstoccasion). Again, alternative constellation values are selected, an IFFTcomputed for each value, and the resulting time-domain multicarriersymbols are monitored. This requires a maximum 8 IFFT operations. Ingeneral,

number of extra IFFT operations=4*number_of_modified_tones.

If a symbol is remapped the buffer containing the symbol with anunacceptable peak is deleted or overwritten with the new (remapped)symbol.

A disadvantage of the above described scheme is that the IFFT block 18is required to operate at a higher rate than normal (e.g. 4 or 8 times).An improvement to the above method will now be described. As before,peak detector 26 detects when a time-domain multicarrier symbol has apeak which is too high. The position of the peak is determined. Asbefore, the tone (or tones) with the lowest number of bits mapped tothem is selected for modifying. The maximum and minimum positions of thetime-domain representation of this carrier are known (see detailedworking below).

The phase of the carrier can be modified to have a peak of oppositepolarity at the required position, i.e. instead of having apositive-going peak (maxima) at time x, the signal has a negative-goingpeak (minima).

Example of a Wired Transmission Real Signal

number of carriers=Nselected carrier=k (and its mirror −k, because it's a real transmission)the maxima (positive peaks) for the first quadrant are the positionswhere:

$\frac{^{j \cdot {({\frac{2 \cdot \pi \cdot k \cdot n}{N} + \frac{\pi}{4}})}} + ^{{- j} \cdot {({\frac{2 \cdot \pi \cdot k \cdot n}{N} + \frac{\pi}{4}})}}}{2} \equiv {{+ \; 1}\mspace{11mu} \text{=}\text{>}\mspace{11mu} {\cos ( {\frac{2 \cdot \pi \cdot k \cdot n}{N} + \frac{\pi}{4}} )}} \equiv {+ 1}$$\begin{matrix} \Rightarrow{{\frac{2 \cdot \pi \cdot k \cdot n}{N} + \frac{\pi}{4}} \equiv {2 \cdot T \cdot \pi}}  & {T\mspace{14mu} {is}\mspace{14mu} {integer}} \\ \Rightarrow{n \equiv {( {T - \frac{1}{8}} ) \cdot \frac{N}{k}}}  & \;\end{matrix}$

For k=4 and N=512 the maxima are found at:

$n = {{\frac{7}{8} \cdot \frac{512}{4}} = 112}$$n = {{\frac{15}{8} \cdot \frac{512}{4}} = 240}$$n = {{\frac{23}{8} \cdot \frac{512}{4}} = 368}$$n = {{\frac{31}{8} \cdot \frac{512}{4}} = 496}$

FIG. 7 shows a time-domain representation of the carrier, with the solidcurve representing the first quadrant. Only two quadrants are shown inFIG. 7 rather than four but this is for only for clarity purposes, i.e.not to overcomplicate this figure. It can be seen that the position ofthe maxima (positive peaks) 71, 72, 73, 74 corresponds to that computedabove. The minima (peaks of negative polarity) are shown at positions75, 76, 77, 78. The broken line represents the second quadrant. Similarto above, it is possible to calculate the position of maxima and minimafor the second quadrant (plotted in FIG. 7) as well as the third andfourth quadrants. The quadrant of the cosine wave is selected which hasa peak closest to the position of the peak in the multicarrier symbol(with the peak of the cosine wave being of the opposite polarity to thepeak in the multicarrier symbol). It is noted that in this method one offour (valid) constellation values are used. As a result of this, theposition of the peak in the cosine wave and the peak in the multicarriersymbol may not always exactly coincide. If the standard governing thetransmission scheme allows a transmitted carrier to have anyconstellation value (i.e. the carrier can have an amplitude and phasewhich does not correspond to any of the amplitude/phase values whichdefine the valid set of constellation values) then it is possible toprovide a peak in the selected carrier at a position which is exactlyaligned with the peak (of opposite polarity) in the multicarrier symbol.

This method requires only one extra IFFT operation since, due to theanalysis of the selected carrier, it is known that the modified carrierwill reduce the peak in the multicarrier symbol. If, following oneiteration of this method, the peak in the multicarrier symbol is stilltoo high a second tone is selected and the optimal phase of the carrieris computed as before. This requires 2 extra IFFT operations. Ingeneral:

number of extra IFFT operations=number_of_modified_tones.

There are possibilities to estimate the number of modified tones, forexample by checking the peak amplitude. This can significantly reducethe number of extra IFFT operations needed. As an example, it may beassumed that every modified tone contributes a peak reduction of 0.4 dbso that:

${{number}\mspace{14mu} {of}\mspace{14mu} {tones}\mspace{14mu} {to}\mspace{14mu} {modify}} = \frac{{required\_ peak}{\_ reduction}}{0.4\mspace{14mu} {dB}}$

The relationship between the number of modified tones and the amount ofpeak reduction can be heuristically determined and stored in a memory ofthe transmitter. The heuristic determination can be made by anindividual transmitter or can be made by the system and results sent toindividual transmitters for storage and use by those transmitters.

In the above methods a multicarrier symbol such as an OFDM symbol ismodified by causing a mapper to output modified constellation value on aparticular carrier, or carriers, and by recalculating the IFFT withthose modified values. An alternative technique will now be describedwhich has an advantage of avoiding the need to recalculate the IFFT asall of the modifying of the multicarrier symbol occurs in thetime-domain, after IFFT block 18. FIG. 8 shows apparatus. As before, apeak detector 26 is positioned after the frequency domain to time domaintransform block, e.g. an IFFT block, and detects a peak in thetime-domain multicarrier symbol. A precomputed time-domainrepresentation of a signal corresponding to a modulated carrier isstored in memory 28, e.g. a stored cosine wave. When a peak is found,the following is carried out:

-   -   a carrier is selected for modifying based on the number of bits        allocated to that carrier;    -   the stored cosine wave is scaled and rotated to reflect the        current constellation value of the selected carrier and is        subtracted from the multicarrier symbol to thereby remove the        selected carrier. It is noted that the constellation value of a        carrier determines the amplitude and phase of the carrier. Since        the mapper 14 has just allocated a constellation value to each        carrier, the constellation modifier 27 can use this information        to scale and rotate the stored cosine wave into the required        position to cancel the selected carrier.    -   the stored cosine wave is, cyclically permuted to align a peak        (of required polarity) in a required position. There are two        possibilities:        -   (i) if the transmission scheme requires valid constellation            values to be transmitted the method is as before. The only            difference is that the multicarrier symbol is generated            directly in the time domain, and does not require an IFFT            operation.        -   (ii) if the transmission scheme does not require valid            constellation values to be transmitted the peak of the            cosine wave can be aligned exactly with the peak in the            multicarrier symbol.    -   the aligned cosine wave is added 29 to the time-domain signal.        Of course, as before, the peak in the cosine wave is of the        opposite polarity to the peak in the multicarrier symbol which        needs to be cancelled (i.e. a negative peak in the cosine wave        is aligned with a positive peak in the multicarrier symbol and a        positive peak in the cosine wave is aligned with a negative peak        in the multicarrier symbol.)

While constellation modifier 27 can store a time-domain representationof each of the carriers used within the transmission scheme, there is asimpler method which requires only one cosine wave to be stored 28. Fromthe single stored cosine wave, a time-domain signal representing acarrier of any carrier frequency, amplitude and phase can be derived.Assume now that the stored cosine wave has a phase of 0rad and anamplitude of 1. In order to match the amplitude a multiplier can beused, in order to match the phase the reading out of the buffer has tobe started at a different position than zero (assume n=512, k=timeindex, f=carrier frequency index, p*pi=phase):

stored_cos(k) = cos (2^(*)pi^(*)k/512) $\begin{matrix}{{{general\_ cos}( {k,f,p} )} = {\cos ( {{2^{*}{pi}^{*}k^{*}{f/512}} + {p^{*}{pi}}} )}} \\{= {\cos ( {2^{*}{{pi}^{*}\lbrack {{k^{*}{f/512}} + {p/2}} \rbrack}} )}} \\{= {\cos ( {2^{*}{{pi}^{*}\lbrack {{k^{*}f} + {256^{*}{p/512}}} \rbrack}} )}} \\{= {{buffer\_ contents}( {{k^{*}f} + {256^{*}p}} )}}\end{matrix}$

Some examples will now be explained in detail to illustrate the effectof the improved methods described above. In this section ADSL is used asan example, but the invention is not limited to ADSL and can also applyto any other multicarrier system, e.g. DMT or OFDM system.

1^(st) Example

Using the bit allocation shown in FIG. 4, the 1 (2) tone(s) having thefewest bits are selected for modifying. Each of these tones carries only2 bits of data. This will result in 2 (4) bits being decoded wrongly.

If the symbol was clipped, as in the prior art, it is very likely thatall tones carrying more than 13 bits are destroyed (tones 9-27). In thisexample this equals 236 bits.

The invention in this case gives rise to ˜120 (˜60) times less corruptedbits than hard clipping. The reason for all tones having >13 bits beingdestroyed is that those tones (carriers) have a larger constellationwhich is more susceptible to corruption.

2^(nd) Example

In a worst case, where all carriers have the maximum (equal) number ofbits allocated to them, 1 (2) modified carrier(s) have 15 bits mapped onthem and so in total 15 (30) bits will be decoded wrongly opposed to 236bits. This is still ˜16 (˜8) times less corrupted bits than hardclipping.

Empirically it is noticed that, in this example, using 1 tone reducesthe crest factor by 0.75 dB and using 2 tones by 1.5 dB. The ADSLstandard requires a BER of 1E-7. With respect to hard clipping thiscould be translated as: maximum 1 symbol out of 1E7 can be clipped inorder not to exceed the standard requirement. FIG. 9 shows the clippingprobability of a typical upstream ADSL signal. The minimum clippingratio should be 5.7 (˜15.1 dB) not to exceed a BER of 1E-7.

There are in general at least two ways of using this invention: eitherreducing the BER for the same clipping ratio or reducing the clippingratio for the same BER. The second option is often the most useful.

Case 1: Same Clipping Ratio, Reduced BER (FIG. 9)

Using one tone reduces the crest factor by about 0.75 dB. This meansthat a clipping probability of 1E-7 corresponds to a clipping ratio of5.2 (˜14.35 dB). From the previous examples it is clear that only afraction of the symbol is corrupted, so the corresponding BER of thefirst and second example (in the case where only 1 tone is used) are8.3E-10 (=1E-7/120) and 6.25E-9 (=1E-7/16) respectively.

Case 2: Same BER, Reduced Clipping Ratio (FIG. 10)

In the first example about 60 symbols out of 1E7 (when using two tones)can be ‘clipped’ in order not to exceed the standard requirement. Theclipping probability (60/1E7) corresponds to a minimum clipping ratio of5.2 (˜14.3 dB). When only one tone is used this corresponds to a minimumclipping ratio of 5.1 (˜14.2 dB), but of course the crest reduction isless (˜0.75 dB). In this example the crest factor has been reduced by 1dB.

The transmitter which has been described above can be implemented as aprogram running on a processing platform. The processing platform can bea general purpose platform such as a personal computer or one which isoptimized to implement the functional elements within the transmitter.The transmitter can be implemented as an integrated circuit whichincludes the processor and memory for storing control instructions tocause the processor to perform the above described tasks. Theinstructions can be arranged as code modules which perform the tasks.The processor can be implemented as an integrated circuit comprising anembedded processor such as a programmable, or reconfigurable, gate arrayor any other suitable processing means.

The invention is not limited to the embodiments described herein, whichmay be modified or varied without departing from the scope of theinvention.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be within the spirit andscope of the invention. Accordingly, the foregoing description is by wayof example only and is not intended as limiting. The invention islimited only as defined in the following claims and the equivalentsthereto.

1-22. (canceled)
 23. A receiver for receiving a modified multicarriersignal provided by a transmitter, the multicarrier signal modified byacts of: applying a frequency domain-to-time domain transform to a setof modulated carriers to generate a time-domain signal; detecting whenthe time-domain signal exceeds a predetermined criterion; and modifyingthe constellation value of at least one of the carriers, wherein the atleast one of the carriers is selected for adjustment on the basis of anumber of bits allocated to the at least one of the carriers.